Avoid using data-sly-test to create variables, prefer data-sly-set

[toniedzwiedz]

data-sly-test had been known as the only way to assign a value to an identifier for a long time until data-sly-set was introduced for that purpose.

Valid use of data-sly-test:

<div class="foo" data-sly-test=${model.renderCondition}>
    <p>${model.text}</p>
</div>

Invalid use of data-sly-test

<sly data-sly-test.profile="${model.profile}" />
<div class="foo">
Name: ${profile.firstName}, Surname: ${profile.secondName}
</div>

Instead, data-sly-set should be used

<div class="foo" data-sly-set.profile="${model.profile}">
Name: ${profile.firstName}, Surname: ${profile.secondName}
</div>

Exception

When data-sly-test is actually used as a test condition, as well as a way to store a value?

<div class="foo" data-sly-test.profile="${model.profile}">
Name: ${profile.firstName}, Surname: ${profile.secondName}
</div>

To be discussed. This is closely related to the existing rule HTL-4 introduced in #131